1 Technical Field
The invention is related to optical communications and in particular to electro-optic modulators having broad band-width.
2. Background Art
Electro-optic modulators of the type which modulate a light beam with an RF or microwave signal using the electro-optic effect are described in U.S. Pat. No. 5,076,655. As described therein, the top surface of a crystalline Lithium Niobate substrate has a series of planar broadside RF antennas overlying an optical waveguide in the substrate. A light beam to be modulated travels through the optical waveguide. A modulating RF or microwave signal is directed toward the series of RF antennas through the substrate at an angle with respect to the top surface corresponding to the arcsin of the ratio of the phase velocities of the light beam and the RF signal in the substrate. The broadside RF antennas direct the RF signal's energy to the light beam, causing the RF signal to phase modulate the light beam in the optical waveguide. The particular angle described above related to the ratio of the phase velocities of the light beam and RF signal compensates for the difference in phase velocities of the light beam and the RF signal so as to maximize the coupling between the RF signal and the light beam.
In the above-referenced patent, a variation of the foregoing is described in which the modulating RF signal or microwave travels from one end of the substrate to the other along the series of broadband RF antennas on the top surface. In this variation, the RF signal or microwave may be thought of as bouncing between the top and bottom surfaces of the substrate at an angle corresponding to the angle discussed above (i.e., the arcsin of the ratio of the phase velocities of the RF signal and the light beam in the substrate), so that the RF signal or microwave does not travel parallel to the longitudinal axis of the substrate.
One disadvantage of the foregoing technique is that the RF broadside antennas employed therein have a relatively narrow band response. A related disadvantage is that a external waveguide (such as an additional Lithium Niobate substrate cut at an angle) must be attached to the bottom of the substrate (in the preferred embodiment thereof) to guide the incoming RF signal through the substrate toward the series of broadside RF antennas on the top surface at the desired angle.
Moreover, in the variation mentioned above in which the RF signal travels along the length of the substrate, the distance between the top and bottom substrate surfaces must be precisely adjusted to the wavelength of the RF signal in order to optimize coupling between the RF signal and the light beam. Such optimization therefore occurs only at a predetermined RF frequency, so that the modulator thereof is a narrow-band device.
There is a need for a wide-band electro-optic modulator which is not limited by the narrow-band response of the broadside RF antennas employed in the conventional electro-optic modulator described above. Moreover, there is a need to simplify the construction of such an electro-optic modulator by eliminating the need for an external waveguide to be attached at right angles to guide the incoming RF signal or microwave at the requisite angle toward the RF antennas.